Magnesium oxide particle aggregate

ABSTRACT

The present invention is related to a magnesium oxide particle aggregate with the requirement of a first inflection point diameter is more than 0.30×10 −6  to 0.60×10 −6  m, a particle void volume is 0.50×10 −3  to 0.90×10 −3  m 3 ·kg −1 , and a micropore volume is 0.04×10 −3  to 0.11×10 −3  m 3 ·kg −1  in the cumulative intrusion volume curve of said particle by having controlled particle aggregation structure so that the solid phase-solid phase reaction between magnesium oxide and the SiO 2  film on the surface can be appropriately controlled.

FIELD OF THE INVENTION

[0001] The present invention relates to a magnesium oxide particleaggregate having a controlled particle aggregation structure. Moreparticularly, the present invention relates to a magnesium oxideparticle aggregate used as an annealing separator to form a forsteritefilm which imparts excellent insulation properties and magneticproperties to a grain-oriented magnetic steel sheet.

BACKGROUND ART

[0002] Grain-oriented magnetic steel sheets used in transformers orgenerators are generally produced by a process in which silicon steelcontaining about 3% of Si is hot-rolled, subsequently cold-rolled so asto have a final sheet thickness, and then subjected to decarburizationannealing (primary recrystallization annealing), followed by finishingannealing. In this process, for imparting insulation properties to amagnetic steel sheet, after the decarburization annealing and before thefinal finishing annealing, a slurry containing magnesium oxide isapplied to a surface of the steel sheet and then dried, and wound into acoil form. Si contained in the silicon steel sheet reacts with oxygenduring the decarburization annealing to form an SiO₂ film on the surfaceof the steel sheet. SiO₂ in the film then reacts with magnesium oxide inthe slurry during the finishing annealing to form a forsterite (Mg₂SiO₄)film having excellent insulation properties on the surface of the steelsheet. The forsterite film is considered to impart not only insulationproperties but also a tension to the surface thereof due to thedifference in the coefficient of thermal expansion between theforsterite film and the steel sheet, thus lowering core loss of thegrain-oriented magnetic steel sheet to improve the magnetic properties.

[0003] Therefore, the forsterite film plays an extremely important rolein the production of grain-oriented magnetic steel sheets, and hence theproperties of magnesium oxide forming the forsterite film directlyaffect the magnetic properties thereof. For this reason, a number ofinventions have been made with respect to the magnesium oxide used as anannealing separator, especially having a controlled citric acid activity(CAA) between magnesium oxide particles and citric acid, and, forexample, Japanese Prov. Patent Publication Nos. 58331/1980, 33138/1994,and 158558/1999 have been disclosed.

[0004] However, CAA merely simulates empirically the reactivity in thesolid phase-solid phase reaction between SiO₂ and magnesium oxide whosereaction actually proceeds on the surface of the magnetic steel sheet,based on the solid phase-liquid phase reaction between magnesium oxideand citric acid. Further, magnesium oxide is often present in the formof particle aggregate in which several powder particles bind togetherand agglomerate, and therefore CAA cannot appropriately evaluate theforsterite formation reaction.

[0005] On the other hand, Japanese Prov. Patent Publication No.46259/1998 discloses an invention made in respect of the fact that thefilm quality varies depending on the state of the pores in magnesiumoxide. In this invention, the pore volume is restricted using aconstant-capacity gas adsorption method, however, in the gas adsorptionmethod, it determines the amount of gas molecules which adsorb onto thepore surfaces present on the particle surfaces. Therefore, only pores asvery small as, for example, 0.1×10⁻⁶ m or less can be measured and thus,it is considered that this method is difficult to apply to the particleaggregation structure having a size of about 1×10⁻⁵ to 1×10⁻⁶ m observedin the actual magnesium oxide particle aggregates, and hence cannotappropriately evaluate the forsterite formation reaction.

[0006] In view of this, the present inventors have found indices whichcan appropriately evaluate the forsterite formation reactivity of amagnesium oxide particle aggregate, and have completed an invention of amagnesium oxide particle aggregate having a particle aggregationstructure specified using the indices (Japanese Patent Application No.2000-132370). Namely, in the cumulative intrusion volume curve of themagnesium oxide particle aggregate, if these are controlled in a rangewhere a first inflection point diameter is 0.30×10⁻⁶ m or less, aninterparticle void volume is 1.40×10⁻³ to 2.20×10⁻³ m³·kg⁻¹, and aparticle void volume is 0.55×10⁻³ to 0.80×10⁻³ m³·kg⁻¹, it is possibleto form forsterite at a satisfactory rate on the surface of an magneticsteel sheet.

[0007] However, the present inventors have found that, with respect tothe range considered to be unsuitable for the forsterite formation inthe above earlier patent application filed by the present inventors, forexample, a range in which the first inflection point diameter exceeds0.30×10⁻⁶ m, there is a possibility that excellent forsterite formationis achieved by further strictly controlling the particle aggregationstructure of the magnesium oxide particle aggregate.

[0008] An object of the present invention is to provide a magnesiumoxide particle aggregate having a particle aggregation structure furthercontrolled so that the forsterite formation rate can be appropriatelycontrolled.

[0009] In addition, another object of the present invention is toprovide an annealing separator for grain-oriented magnetic steel sheet,using the magnesium oxide particle aggregate of the present invention,and to provide a grain-oriented magnetic steel sheet obtainable by atreatment using the annealing separator for grain-oriented magneticsteel sheet of the present invention.

DISCLOSURE OF THE INVENTION

[0010] Namely, the present invention is a magnesium oxide particleaggregate characterized in that a first inflection point diameter ismore than 0.30×10⁻⁶ to 0.60×10⁻⁶ m, a particle void volume is 0.50×10⁻³to 0.90×10⁻³ m³·kg⁻¹, and a micropore volume is 0.04×10⁻³ to 0.11×10⁻³m³·kg⁻¹ in the cumulative intrusion volume curve of the particle.

[0011] In addition, the present invention is a magnesium oxide particleaggregate characterized in that an interparticle void volume in thecumulative intrusion volume curve of the particle is 0.80×10⁻³ to lessthan 1.40×10⁻³ m³·kg⁻¹, a particle void volume is 0.50×10⁻³ to 0.90×10⁻³m³·kg⁻¹, and a micropore volume is 0.04×10⁻³ to 0.11×10⁻³ m³·kg⁻¹ in thecumulative intrusion volume curve of the particle.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 shows examples of cumulative intrusion volume curves ofparticle aggregates comprised mainly of magnesium oxide, determined froma pore distribution measurement by mercury porosimetry.

[0013]FIG. 2 shows a relationship between a forsterite formation rate, afirst inflection point diameter, and a particle void volume with respectto various magnesium oxide particle aggregates.

[0014]FIG. 3 shows a relationship between a forsterite formation rate,an interparticle void volume, and a particle void volume with respect tovarious magnesium oxide particle aggregates.

[0015]FIG. 4 shows a relationship between a micropore volume and aforsterite formation rate with respect to the particle aggregate havinga first inflection point diameter of more than 0.30×10⁻⁶ to 0.60×10⁻⁶ mand a particle void volume of 0.50×10⁻³ to 0.90×10⁻³ m³·kg⁻¹.

[0016]FIG. 5 shows a relationship between a micropore volume and aforsterite formation rate with respect to a particle aggregate having aninterparticle void volume of 0.80×10⁻³ to less than 1.40×10⁻³ m³·kg⁻¹and a particle void volume of 0.50×10⁻³ to 0.90×10⁻³ m³·kg⁻¹.

[0017]FIG. 6 shows temperature and time conditions suitable forcontrolling the reaction so that the first inflection point diameterbecomes more than 0.30×10⁻⁶ to 0.60×10⁻⁶ m when magnesium hydroxide isprepared by reacting an aqueous solution of magnesium chloride withcalcium hydroxide.

BEST MODE FOR CARRYING OUT THE INVENTION

[0018] In the present invention, a cumulative intrusion volume curve ofparticles refers to a curve which shows a relationship between a porediameter and a cumulative pore volume determined from a poredistribution measurement by mercury porosimetry, and FIG. 1 showsexamples of cumulative intrusion volume curves of two types of magnesiumoxide particle aggregates having different particle aggregationstructures. The first inflection point is the inflection point at thelargest pore diameter among inflection points at which the cumulativeintrusion volume curve suddenly rises, and it is indicated by a solidcircle in the figure. The first inflection point diameter refers to apore diameter at the first inflection point. The interparticle voidvolume refers to a cumulative pore volume at the first inflection point.The particle void volume is represented by a volume value obtained bysubtracting the cumulative pore volume at the first inflection pointfrom the total pore volume represented by the cumulative pore volume ata pore diameter of 0.003×10⁻⁶ m. The micropore volume refers to acumulative pore volume of micropores smaller than 0.05×10⁻⁶ m, and isrepresented by a volume value obtained by subtracting the cumulativepore volume at 0.05×10⁻⁶ m from the total pore volume and falls in theregion indicated by arrows near the right-hand ordinate in FIG. 1.

[0019] The present inventors have made studies on a solid phase-solidphase reaction between magnesium oxide and SiO₂, which reaction proceedson a surface of a grain-oriented magnetic steel sheet, and, as a result,they have found that the first inflection point diameter in thecumulative intrusion volume curve as determined from a pore distributionmeasurement by mercury porosimetry, a particle void volume, and ainterparticle void volume as well as a micropore volume can be used asindices for properly indicating the structure of a magnesium oxideparticle aggregate. Based on the above finding, these indices arecontrolled so as to fall in respective appropriate ranges to create amagnesium oxide particle aggregate which can appropriately control aforsterite formation on a surface of a grain-oriented magnetic steelsheet.

[0020] The pore distribution measurement by mercury porosimetry forobtaining indices indicating a particle aggregation structure wasconducted by the following method. In the measurements of poredistribution of porous solid materials, the method of mercuryporosimetry is well known as an analysis method for obtaining poredistribution data of powder and data about a particle aggregationstructure.

[0021] As a mercury porosimeter, AutoPore 9410, manufactured byMicromeritics GbmH, is used. Measurement cell for powdery sample havinga cell capacity of 5×10⁻⁶ m³ and a stem capacity of 0.38×10⁻⁶ m³ isused. A sample to be measured is preliminarily treated using a 330 meshstandard sieve (JIS-R8801-87) so as to have substantially uniformparticle diameters and then precisely weighed in the mass range of from0.10×10⁻³ to 0.13×10⁻³ kg, and placed in the measurement cell. Themeasurement cell is set in the porosimeter, and then the inside of thecell is maintained in a reduced pressure state at 50 μHg (6.67 Pa) orless for 20 minutes. Next, mercury is charged into the measurement celluntil the pressure in the cell becomes 1.5 psia (10,342 Pa). Then, themercury is pressed under a pressure in the range of from 2 psia (13,790Pa) to 60,000 psia (413.7 MPa) to measure pore distribution. As themercury for the measurement, a special grade mercury reagent having apurity of 99.5 mass % or higher is used, and the density of the mercuryused is 13.5335×10³ kg·m⁻³.

[0022] The mercury pressing pressure is converted to a pore diameterusing the following formula (I)(Washburn's equation).

D=−(1/P)·4γ·cos φ  (I)

[0023] wherein D: pore diameter (m);

[0024] P: mercury pressing pressure (Pa);

[0025] γ: surface tension of mercury {485 dyne·cm⁻¹ (0.485 Pa·m)}; and

[0026] φ: contact angle of mercury (130°=2.26893 rad).

[0027] When pressing mercury into a particle aggregate, mercury firstpenetrates into the voids between the particles. In this instance, asthe mercury pressing pressure increases, that is, the pore diameterdetermined from the mercury pressing pressure decreases, the cumulativepore volume increases with a substantially constant gradient. After allvoids between the particles are filled with mercury, mercury startspenetrating into the voids in the particles. A great number of voidshaving substantially the same size are present in the particles, and thesum of the voids in the particles (the sum of the pore volumes) islarge. Therefore, when the penetration of mercury is changed fromthrough the voids between the particles to through the voids in theparticles, the cumulative pore volume drastically increases even as themercury pressing pressure slightly increases. This can be seen in thecumulative intrusion volume curves of FIG. 1.

[0028] A first inflection point diameter, a particle void volume, aninterparticle void volume, and a micropore volume are individuallydetermined from the cumulative intrusion volume curve as follows.

[0029] In the cumulative intrusion volume curves of FIG. 1, thecumulative pore volume on the ordinate indicates a cumulative value ofthe pore volume in the particle aggregate per unit mass of the sampledetermined from larger pore diameters successively. The inflection pointis a point at which a cumulative intrusion volume curve suddenly rises.The number of inflection point is not necessarily one, and, depending onthe sample to be measured, as can be seen in curve B in FIG. 1, there isthe case where a plurality of inflection points are present, but theinflection point at the largest pore diameter is taken as the firstinflection point. The first inflection point diameter is the porediameter at the first inflection point. The interparticle void volume isa void volume between the aggregate particles, and it is represented bythe cumulative pore volume at the first inflection point. The particlevoid volume is a void volume which is present in the particles andsmaller than the diameter of the aggregate particles, and it isrepresented by a volume value obtained by subtracting the cumulativepore volume at the first inflection point from the total pore volume.The micropore volume is represented by a volume value obtained bysubtracting the cumulative pore volume at a pore diameter of 0.05×10⁻⁶ mfrom the total pore volume. The total pore volume is a cumulative porevolume at a pore diameter of 0.003×10⁻⁶ m. This is because the particlestructure is changed due to the mercury pressing pressure in the poredistribution measurement by mercury porosimetry, and therefore, themeasurement error can be lowered by using the cumulative pore volume atthe maximum mercury pressing pressure as a total pore volume.

[0030] Next, magnesium oxide particle aggregates having differentparticle aggregation structures, in which a first inflection pointdiameter, a particle void volume, a interparticle void volume, and amicropore volume in a cumulative intrusion volume curve are individuallydifferent, were prepared to examine the reaction rates of the solidphase reactions between the individual magnesium oxide particleaggregates and SiO₂. The results are shown in Table 1. TABLE 1Forsterite First inflection Particle Interparticle Micropore formationpoint diameter void volume void volume volume rate Unit *10⁻³ *10⁻³*10⁻³ *10⁻⁶ m m³ · kg⁻¹ m³ · kg⁻¹ m³ · kg⁻¹ % A 0.17 0.77 2.16 0.04 91.3B 0.18 0.88 1.31 0.02 86.2 C 0.17 0.83 2.44 0.08 84.7 D 0.23 0.65 1.680.05 91.9 E 0.23 0.66 1.35 0.03 89.7 F 0.24 0.65 2.33 0.01 86.5 G 0.210.85 1.81 0.03 89.5 H 0.22 0.58 1.43 0.01 90.5 I 0.25 0.56 2.12 0.0190.1 J 0.22 0.52 1.59 0.01 86.8 K 0.27 0.74 1.46 0.05 91.8 L 0.26 0.491.31 0.02 81.2 M 0.27 0.53 2.27 0.05 82.1 N 0.45 0.71 1.51 0.03 87.2 O0.43 0.72 1.28 0.01 84.9 P 0.44 0.72 2.29 0.02 83.6 Q 0.43 0.91 1.620.02 81.0 R 0.42 0.48 1.45 0.01 78.8 S 0.59 0.52 2.25 0.01 77.0 T 0.610.48 1.34 0 75.5 U 0.88 0.84 1.27 0 77.7 V 0.90 0.85 2.32 0.03 75.6 a0.38 0.61 0.85 0.08 92.5 b 0.55 0.82 1.29 0.10 91.9 c 0.42 0.72 1.880.05 90.7 d 0.28 0.66 0.89 0.05 90.6 e 0.67 0.58 1.45 0.06 87.6 f 0.950.77 1.89 0.10 79.3 g 0.41 0.54 1.24 0.02 85.6 h 0.35 0.72 1.42 0.1387.3 i 0.45 0.45 1.37 0.07 85.4 j 0.39 0.94 1.28 0.09 86.1 k 0.24 0.620.75 0.01 82.3 l 0.45 0.71 2.33 0.03 84.6 m 0.54 0.82 0.87 0.18 83.1

[0031] In the solid phase-solid phase reaction between magnesium oxideand SiO₂, these were directly reacted with each other to formforsterite. Namely, each magnesium oxide particle aggregate having anindividual particle aggregation structure and amorphous SiO₂ were mixedin a molar ratio of 2:1 to form a mixture, and it was press-molded undera pressure of 50 MPa into pellets having a diameter of 15×10⁻³ m and aheight of 15×10⁻³ m. Then, the molded articles were calcined in anitrogen gas atmosphere at 1,200° C. for 4 hours. With respect to thethus obtained sintered product, an X-ray diffraction analysis wasconducted to quantitatively determine a forsterite formation rate in thesintered product.

[0032] With respect to the thus measured samples, a relationshipsbetween a forsterite formation rate, a first inflection point diameter,a particle void volume, a interparticle void volume, and a microporevolume in a cumulative intrusion volume curve are shown in FIGS. 2 to 5.In FIGS. 2 and 3, forsterite formation rate values are indicated byclassifying into three levels, i.e., 90% or more, more than 80 to lessthan 90%, and less than 80%. In the figures, area a sectioned by adotted line corresponds to the range defined in the above-mentionedearlier patent application filed by the present inventors, and area bsectioned by a solid line corresponds to the range defined in thepresent invention.

[0033] The first inflection point diameter indicates the size of thelargest particle structure in the aggregate particles, and, the smallerthe first inflection point diameter, the larger the number of contactpoints between the magnesium oxide particles and SiO₂, or the higher theactivity. Therefore, the first inflection point diameter is preferably0.30×10⁻⁶ m or less, which corresponds to the range in the earlierpatent application filed by the present inventors, and, when the firstinflection point diameter exceeds 0.30×10⁻⁶ m, the number of contactpoints between the magnesium oxide particles and SiO₂ generally lacks,making it difficult to form forsterite at a satisfactory rate. However,as can be seen in FIG. 2, there is a specific region such thatforsterite can be formed at a rate of 90% or higher at a firstinflection point diameter of 0.60×10⁻⁶ m or less even when the firstinflection point diameter exceeds 0.30×10⁻⁶ m. This region correspondsto the case where the particle void volume falls in the range of0.50×10⁻³ to 0.90×10⁻³ m³ kg⁻¹ (FIG. 2) and the micropore volume fallsin the range of 0.04×10⁻³ to 0.11×10⁻³ m³·kg⁻¹ (FIG. 4). The reason forthis is presumed that the highly active magnesium oxide particlesurfaces which are partially present on the particles make up for thelack of the contact points in this region. With respect to theforsterite formation, a value of 90% or more was taken as a satisfactoryvalue for the forsterite formation rate. When the forsterite formationrate can satisfy such a reference value, a forsterite film havingexcellent adhesion to the surface of a magnetic steel sheet can beformed.

[0034] On the other hand, the interparticle void volume is an indexwhich indirectly indicates the form of the aggregate particles, and,when the interparticle void volume falls in an appropriate range, thecontact points between the magnesium oxide particles and SiO₂ can beappropriately controlled. Therefore, when the interparticle void volumeis less than 1.40×10⁻³ m³·kg⁻¹, which corresponds to the range in theearlier patent application filed by the present inventors, the number ofcontact points between the magnesium oxide particles and SiO₂ generallylacks, making it difficult to form forsterite at a satisfactory rate.However, as can be seen in FIG. 3, there is a region such thatforsterite can be formed at a rate of 90% or higher at an interparticlevoid volume of 0.80×10⁻³ m³·kg¹ or more even when the interparticle voidvolume is less than 1.40×10⁻³ m³·kg⁻¹. This region corresponds to thecase where both the particle void volume and the micropore volume fallin the above respective ranges shown in FIGS. 3 and 5.

[0035] In other words, even when the magnesium oxide particle aggregatehas a first inflection point diameter and an interparticle void volumefalling in the respective ranges which are not suitable for theforsterite formation in the earlier patent application filed by thepresent inventors, forsterite is stably formed at a rate of 90% orhigher as long as both the particle void volume and the micropore volumefall in the respective appropriate ranges.

[0036] Namely, the particle void volume is an index indicating thedensity of the aggregate particle. When the particle void volume is lessthan 0.50×10⁻³ m³·kg⁻¹, the number of contact points lacks, and, whenthe particle void volume exceeds 0.90×10⁻³ m³·kg⁻¹, the probability ofcontact between the magnesium oxide particle aggregates becomes toolarge, and hence magnesium oxide undergoes a reaction therebetweenbefore forming forsterite and is then inactivated. Thus, in any cases,forsterite is not formed at a satisfactory rate.

[0037] Further, the micropore volume is critical. The smaller theparticles, the larger the curvature of the particle surface or thehigher the surface energy, then it makes the activity higher. Therefore,it is considered that the particle structure having micropores formedtherein is a highly reactive particle structure. However, when such ahighly active particle structure has a micropore volume of more than0.11×10⁻³ m³·kg⁻¹, the probability of contact between the highly activeparticle structures becomes too large, and hence the highly activeparticle structures undergo a reaction therebetween before contributingto the formation of forsterite, so that forsterite is not formed at asatisfactory rate. On the other hand, when the highly active particlestructure has a micropore volume of less than 0.04×10⁻³ m³·kg⁻¹, thelack of the contact points cannot be made up for, so that forsteritecannot be formed at a satisfactory rate.

[0038] From the above results, when a requirement that the firstinflection point diameter be more than 0.30×10⁻⁶ to 0.60×10⁻⁶ m or theinterparticle void volume be 0.80×10⁻³ to less than 1.40×10⁻³ m³·kg⁻¹,the particle void volume be 0.50×10⁻³ to 0.90×10⁻³ m³·kg⁻¹, and themicropore volume be 0.04×10⁻³ to 0.11×10⁻³ m³ kg⁻¹ is satisfied,forsterite can be formed from the magnesium oxide particle aggregate andSiO₂ stably at a rate of 90% or higher.

[0039] Next, a magnesium oxide particle aggregate having a firstinflection point diameter, an interparticle void volume, a particle voidvolume, and a micropore volume each being in the above-mentioned rangecan be prepared as follows. It is noted that the preparation methoddescribed below is merely one example, and a magnesium oxide particleaggregate having the particle aggregation structure defined in thepresent invention can be prepared by other methods.

[0040] The magnesium oxide particle aggregate can be prepared asfollows. For example, calcium hydroxide is added to an aqueous solutionof magnesium chloride as a raw material to form magnesium hydroxide, andthen the magnesium hydroxide is subjected to filtration by means of afilter press, and dehydrated and dried, and then calcined using a rotarykiln, followed by grinding.

[0041] The magnesium oxide particle aggregate can be prepared by variousmethods, for example, a method in which an alkaline aqueous solution,such as an aqueous solution of calcium hydroxide, sodium hydroxide, orpotassium hydroxide, is reacted with a magnesium chloride-containingaqueous solution, such as bittern, brackish water, or sea water, toobtain magnesium hydroxide, and the magnesium hydroxide is calcined toobtain a magnesium oxide particle aggregate; a method in which magnesiteis calcined to obtain a magnesium oxide particle aggregate; a method(Aman method) in which a magnesium oxide particle aggregate is directlyobtained from a magnesium chloride-containing aqueous solution; and amethod in which magnesium oxide obtained by the above method issubjected to hydration to form magnesium hydroxide, followed bycalcination, to obtain a magnesium oxide particle aggregate.

[0042] The first inflection point diameter and particle void volume ofthe magnesium oxide particle aggregate can be adjusted by controllingthe particle structure of magnesium hydroxide which is a precursor ofmagnesium oxide, the interparticle void volume can be adjusted bycontrolling the conditions for grinding magnesium oxide obtained bycalcination, and the particle structure having micropores formed thereincan be obtained by mixing particle aggregates having differentcalcination degrees prepared by calcining magnesium hydroxide particleshaving a specific size or larger (BET specific surface area of 15×10³m²·kg⁻¹ or less).

[0043] The first inflection point diameter and particle void volume ofthe magnesium oxide particle aggregate are adjusted by controlling theparticle structure of magnesium hydroxide which is a precursor ofmagnesium oxide. Namely, a calcium hydroxide slurry is added to amagnesium chloride solution so that the resultant magnesium hydroxideconcentration becomes a predetermined value, and the resultant mixtureis stirred to effect a reaction at a predetermined temperature for apredetermined time, and then, the reaction mixture is subjected tofiltration by means of a filter press, and washed with water and driedto form magnesium hydroxide.

[0044] For adjusting the first inflection point diameter to be more than0.30×10⁻⁶ to 0.60×10⁻⁶ m, the reaction temperature and reaction time forthe magnesium hydroxide formation are controlled. Namely, as shown inFIG. 6, magnesium hydroxide is formed by a reaction under conditionssuch that the reaction temperature (T, ° C.) and the reaction time (t,hr) satisfy the relationship represented by the following formula (II).

(1.033×10⁵)exp{(−8.5×10⁻²)·T}≦t≦(7.861×10⁶)exp{(−8.32×10⁻²)·T}  (II)

[0045] Further, it is more preferred that the reaction temperature (T, °C.) and the reaction time (t, hr) satisfy the relationship representedby the following formula (III).

(1.943×10⁶)exp{(−8.45×10⁻²)·T}≦t≦(2.180×10⁶)exp{(−8.35×10⁻²)·T}  (III)

[0046] The particle void volume is adjusted by controlling the magnesiumhydroxide concentration after the reaction. Namely, the ratio betweenthe magnesium chloride solution and the calcium hydroxide slurry mixedis adjusted so that the magnesium hydroxide concentration after thereaction becomes 0.2 to 4.5 mol·kg⁻¹, preferably 0.5 to 3 mol·kg⁻¹.

[0047] The micropore volume is adjusted by controlling the calcinationconditions (temperature×time) for the magnesium hydroxide formed, andmagnesium oxide particle aggregates having different calcination degreesare prepared and mixed with each other. In this case, the calcinationtemperature is 750 to 1,250° C., and the calcination time is 0.2 to 5hours. In the mixing, with respect to each of the magnesium oxideparticle aggregates obtained by calcination under specific calcinationconditions, a micropore volume is measured using a mercury porosimetrycurve and a mixing ratio is determined by making calculation, and then amixture having a specific micropore volume is obtained.

[0048] For obtaining a particle structure having micropores formedtherein, it is required to form by calcination magnesium hydroxideparticles having a specific size or larger such that the BET specificsurface area is 15×10³ m²·kg⁻¹ or less. The crystal of magnesiumhydroxide is of trigonal crystal system, and generally has a hexagonalplate form. In the present invention, the magnesium hydroxide particlesmay be either single crystalline or polycrystalline, and the form of theparticles is not limited to the hexagonal plate form, but the size ofthe magnesium hydroxide particles indicated by the BET specific surfacearea is important. The reason for this resides in that, when the BETspecific surface area of the magnesium hydroxide particles exceeds15×10³ m²·kg⁻¹, it is difficult to obtain a particle structure havingmicropores required. Namely, when magnesium hydroxide changes tomagnesium oxide, a volume shrinkage of 50% or more occurs, andtherefore, in the smaller magnesium hydroxide particles, the magnesiumoxide particles formed moves due to deformation caused by the shrinkageto form relatively large magnesium oxide particles, thus making itdifficult to form small magnesium oxide particles needed to formmicropores in a specific range.

[0049] The interparticle void volume is adjusted by controlling thegrinding of the calcined magnesium oxide. For example, when grindingusing a hammer mill-type grinder at a power of 5.5 kW having aclassifier, the hammer rotational frequency is preferably 2,800 rpm orless.

[0050] As a grinder, a hammer mill-type grinder, a high-speed rotatingmill-type grinder, a jet mill-type grinder, a roller mill-type grinder,or a ball mill-type grinder can be used. The optimal conditions of thegrinder for obtaining the interparticle void volume which falls in therange defined in the present invention vary depending on the system andability (power) of the grinder used, but too strong grinding increasesthe interparticle void volume and too weak grinding lowers theinterparticle void volume. In the jet mill-type grinder in which theimpact energy applied during grinding is large, the impact energy maylower the particle void volume, and therefore the operation of thegrinder of this type needs to be controlled under conditions suitablefor the apparatus. Further, a classifier is not necessarily used, butthe use of a classifier makes it possible to control the grindingconditions more flexibly.

[0051] In the reaction of magnesium hydroxide, a flocculant can be addedfor promoting the aggregation reaction, and a flocculation preventingagent can be added for preventing the aggregation reaction fromproceeding to an excess extent. Examples of flocculants include aluminumsulfate, polyaluminum chloride, iron sulfate, and polyacrylamide, andpreferred are polyaluminum chloride and anionic polyacrylamide. Theflocculant can be added in an amount of 1 to 1,000 ppm, preferably 5 to500 ppm, more preferably 10 to 100 ppm, based on the total mass of themagnesium chloride solution and the calcium hydroxide slurry. It is notpreferred to add a flocculant in an excess amount since a particleaggregate having too high a density such that the particle void volumeis less than 0.50×10⁻³ m³·kg⁻¹ is disadvantageously formed.

[0052] On the other hand, as a flocculation preventing agent, sodiumsilicate, sodium polyphosphate, sodium hexametaphosphate, a nonionicsurfactant, or an anionic surfactant can be added, and preferred aresodium silicate, sodium hexametaphosphate, and nonionic surfactants. Theflocculation preventing agent can be added in an amount of 1 to 1,000ppm, preferably 5 to 500 ppm, more preferably 10 to 100 ppm, based onthe total mass of the magnesium chloride solution and the calciumhydroxide slurry. It is not preferred to add a flocculation preventingagent in an excess amount since a particle aggregate having such a lowdensity that the particle void volume is more than 0.90×10⁻³ m³·kg⁻¹ isdisadvantageously formed.

[0053] A calcium hydroxide slurry is added to a magnesium chloridesolution, and then the stirring was conducted at a stirring rate of 350to 450 rpm. The stirring does not largely affect the particle structure,but the interparticle void volume can be increased by stirring at a highspeed and at a high shear rate by means of, for example, a homogenizerduring the reaction or can be lowered by almost no stirring.

[0054] Next, using the thus obtained magnesium oxide, an annealingseparator for grain-oriented magnetic steel sheet and a grain-orientedmagnetic steel sheet are produced as follows.

[0055] A grain-oriented magnetic steel sheet is produced as follows: asilicon steel slab having an Si content of 2.5 to 4.5% is hot-rolled andwashed with an acid, and then subjected to cold rolling or twice coldrolling including intermediate annealing so that the resultant sheet hasa predetermined thickness. Then, the cold-rolled coil is subjected torecrystallization annealing, which also effects decarburization, in awet hydrogen gas atmosphere at 700 to 900° C. to form an oxide filmcomprised mainly of silica (SiO₂) on the surface of the steel sheet. Anaqueous slurry obtained by uniformly dispersing in water the magnesiumoxide particle aggregate having the particle aggregation structure ofthe present invention prepared by the above method is continuouslyapplied onto the resultant steel sheet using a roll coater or a spray,and dried at about 300° C. The thus treated steel sheet coil issubjected to final finishing annealing, for example, at 1,200° C. for 20hours to form forsterite (Mg₂SiO₄) on the surface of the steel sheet,and the forsterite imparts a tension to the surface of the steel sheetalong with the insulating film to improve the core loss ofgrain-oriented magnetic steel sheet.

[0056] As described in, for example, Japanese Prov. Patent PublicationNo. 101059/1994, for facilitating the forsterite film formation, a knownreaction accelerator, inhibitor auxiliary, or tension-impartinginsulating film additive can be added to the annealing separator.

EXAMPLES

[0057] Next, the present invention will be described with reference tothe following Examples.

Example 1

[0058] A calcium hydroxide slurry was added to a magnesium chloridesolution having a concentration of 2.0 mol·kg⁻¹ so that the magnesiumhydroxide concentration after reaction became 1.2 mol·kg⁻¹, and theresultant mixture was subjected to reaction in an autoclave at 150° C.for 3 hours to prepare magnesium hydroxide having a BET specific surfacearea of 8.2×10³ m²·kg⁻¹. The magnesium hydroxide prepared was calcinedusing a rotary kiln individually at temperatures of 800° C., 950° C.,and 1,050° C. for one hour, and then ground by means of an impactgrinder to produce three types of magnesium oxide particle aggregateshaving different calcination degrees. Then, the three types of magnesiumoxide particle aggregates produced were mixed together in a mixing ratioof 30:40:30 to obtain a magnesium oxide particle aggregate having aparticle aggregation form which falls in the range defined in thepresent invention.

Example 2

[0059] Magnesite was calcined using a rotary kiln at 1,100° C. for onehour to prepare magnesium oxide having a BET specific surface area of5.2×10³ m²·kg⁻¹. The magnesium oxide prepared was added to water so thatthe slurry concentration became 2 mol·kg⁻¹ to effect a reaction at 90°C. for 2 hours, preparing magnesium hydroxide having a BET specificsurface area of 7.5×10³ m²·kg⁻¹. Then, the magnesium hydroxide preparedwas calcined using a rotary kiln at 980° C. individually for 0.2 hour,0.5 hour, 0.8 hour, and 2 hours, and then ground by means of an impactgrinder to produce magnesium oxide particle aggregates having differentcalcination degrees. Then, the four types of magnesium oxide particleaggregates produced were mixed together in a mixing ratio of 25:30:15:30to obtain a magnesium oxide particle aggregate in Example 2 having aparticle aggregation form which falls in the range defined in thepresent invention.

Example 3

[0060] A slaked lime slurry was added to bittern so that the magnesiumhydroxide concentration after reaction became 1.2 mol·kg⁻¹, and theresultant mixture was stirred at 600 rpm to effect a reaction at 80° C.for 2 hours. Then, the reaction mixture was subjected to filtration bymeans of a filter press, and washed with water and dried, and theresultant magnesium hydroxide was calcined using a rotary kiln at 900°C. for one hour to prepare magnesium oxide having a BET specific surfacearea of 20.6×10³ m²·kg⁻¹. The magnesium oxide prepared was added towater so that the slurry concentration became 3 mol-kg⁻¹, and thencalcium chloride was added thereto in an amount of 2 mol %, based on themole of the magnesium oxide, and the resultant mixture was subjected toreaction at 80° C. for 2 hours to prepare magnesium hydroxide having aBET specific surface area of 11.0×10³ m²·kg⁻¹. Next, the magnesiumhydroxide was calcined using a muffle furnace at a furnace temperatureof 1,200° C. individually for calcination times of 2 hours, 3 hours, and4 hours, and then ground by means of an impact grinder to producemagnesium oxide particle aggregates having different calcinationdegrees. Then, the three types of magnesium oxide particle aggregatesproduced were mixed together in a mixing ratio of 25:40:35 to obtain amagnesium oxide particle aggregate in Example 3 having a particleaggregation form which falls in the range defined in the presentinvention.

Comparative Example 1

[0061] A slaked lime slurry was added to bittern so that the magnesiumhydroxide concentration after reaction became 2 mol·kg⁻¹, and theresultant mixture was stirred at 600 rpm to effect a reaction at 80° C.for 2 hours. Then, the reaction mixture was subjected to filtration bymeans of a filter press, and washed with water and dried, and theresultant magnesium hydroxide was calcined using a rotary kiln at 890°C. for one hour to prepare a magnesium oxide particle aggregate. Then,the particle aggregate prepared was ground by means of an impact grinderto produce a magnesium oxide particle aggregate in Comparative Example 1having a specific particle aggregation structure in the earlier patentapplication filed by the present inventors (Japanese Patent ApplicationNo. 2000-132370).

Comparative Examples 2 to 4

[0062] The magnesium oxide particle aggregates obtained in Example 1 bycalcining magnesium hydroxide using a rotary kiln individually attemperatures of 800° C., 950° C., and 1,050° C. for one hour, and thengrinding the calcined product by means of an impact grinder were notmixed together but individually used.

Comparative Example 5

[0063] Bittern and slaked lime were reacted with each other at 40° C.for 10 hours to form magnesium hydroxide, and then the magnesiumhydroxide was calcined by means of a rotary kiln at 1,050° C. The thusproduced magnesium oxide particles are not controlled with respect tothe particle aggregation structure as conducted in the presentinvention, and they are magnesium oxide for annealing separator used forgeneral magnetic steel sheets.

Comparative Example 6

[0064] Slaked lime was added to sea water so that the magnesiumhydroxide concentration after reaction became 0.05 mol·kg⁻¹ to effect areaction at 50° C. for 20 hours, thus forming magnesium hydroxide. 5Hours before completion of the reaction, anionic polyacrylamide wasadded in an amount of 200 ppm, and then the reaction mixture aftercompletion of the reaction was subjected to filtration by means of afilter press and dried. Then, the resultant magnesium hydroxide wascalcined by means of a rotary kiln at 950° C. to prepare magnesium oxideparticles. The thus prepared particles are not controlled with respectto the particle aggregation structure as conducted in the presentinvention, and they are magnesium oxide used in an application otherthan annealing separator.

[0065] Table 2 shows the measurement values for particle aggregationstructures of the particles or particle aggregates in Examples 1 to 3and Comparative Examples 1 to 6. As can be seen from this table, in eachof Examples 1 to 3 in which the particle aggregate was produced whilecontrolling the particle aggregation structure, the requirement of thepresent invention that the first inflection point diameter be more than0.30×10⁻⁶ to 0.60×10⁻⁶ m or the interparticle void volume be 0.80×10⁻³to less than 1.40×10⁻³ m³·kg⁻¹, the particle void volume be 0.50×10⁻³ to0.90×10⁻³ m³·kg⁻¹, and the micropore volume be 0.04×10⁻³ to 0.11×10⁻³m³·kg⁻¹ is satisfied. In Comparative Example 1, the first inflectionpoint diameter is 0.30×10⁻⁶ m or less, the interparticle void volume is1.40×10⁻³ to 2.20×10⁻³ m³·kg⁻¹, and the particle void volume is0.55×10⁻³ to 0.80×10⁻³ m³ kg⁻¹, namely, the particle aggregate structurefalls in the range defined in the earlier patent application filed bythe present inventors. On the other hand, in each of ComparativeExamples 2 to 4, the first inflection point diameter, the interparticlevoid volume, and the particle void volume fall in the respective rangesdefined in the present invention, but the micropore volume in Example 2exceeds the upper limit of the range defined in the present invention,and the micropore volume in each of Examples 3 and 4 is less than thelower limit. Further, in each of Examples 5 and 6, the particleaggregation structure is not controlled, and hence, the interparticlevoid volume in Example 5 and the first inflection point diameter inExample 6 fall outside of the respective ranges defined in the presentinvention, and almost no micropores are present in the particles. TABLE2 Measurement values for particle aggregation structures Firstinflection point Interparticle Particle Micropore diameter void volumevoid volume volume Unit *10⁻⁶ m *10⁻³ m³ · kg⁻¹ *10⁻³ m³ · kg⁻¹ *10⁻³ m³· kg⁻¹ Example 1 0.38 1.26 0.74 0.09 Example 2 0.41 1.19 0.69 0.06Example 3 0.29 1.13 0.66 0.10 Com- 0.28 1.45 0.69 0.01 parative Example1 Com- 0.32 1.18 0.75 0.25 parative Example 2 Com- 0.35 1.25 0.77 0.02parative Example 3 Com- 0.55 1.38 0.72 0.01 parative Example 4 Com- 0.352.63 0.84 0.00 parative Example 5 Com- 1.17 0.52 0.62 0.00 parativeExample 6

[0066] Next, with respect to the above magnesium oxide particleaggregates or powder particles, the behavior of formation of aforsterite film was examined. It is presumed that the formation offorsterite proceeds according to the solid phase reaction:2MgO+SiO₂→Mg₂SO₄. Therefore, the magnesium oxide powder in each ofExamples and Comparative Examples and SiO₂ were mixed in a molar ratioof 2:1 to form a mixture, and the mixture was shaped under a pressure of50 MPa to obtain a shaped article having a diameter of 15×10⁻³ m and aheight of 15×10⁻³ m, and then the shaped article was calcined in anitrogen gas atmosphere at 1,200° C. for 4 hours. This calcinationtemperature corresponds to the temperature of the finishing annealing inwhich SiO₂ is reacted with a slurry containing magnesium oxide on thegrain-oriented magnetic steel sheet. With respect to the thus obtainedsintered product, an X-ray diffraction analysis was conducted toquantitatively determine a forsterite (Mg₂SiO₄) formation rate. Theresults are shown in Table 3. TABLE 3 Mg₂SiO₄ formation rate Mg₂SiO₄formation rate/mass % Example 1 93.8 Example 2 91.8 Example 3 92.5Comparative Example 1 90.6 Comparative Example 2 85.6 ComparativeExample 3 83.3 Comparative Example 4 73.4 Comparative Example 5 77.5Comparative Example 6 63.4

[0067] As can be seen from Table 3, in each of Examples 1 to 3, theforsterite formation rate is higher than 90%. In Comparative Example 1which falls in the range in the earlier patent application filed by thepresent inventors, the micropore volume is extremely small, but theforsterite formation rate is higher than 90%. Thus, in this case, evenwhen no micropore is present, forsterite can be formed at a satisfactoryrate. However, in each of Comparative Examples 2 to 5, the forsteriteformation rate is as unsatisfactory as less than 90%. Of these, theforsterite formation rate of the magnesium oxide in Comparative Example5 of an annealing separator in general use and that of the magnesiumoxide in Comparative Example 6 used in an application other than theannealing separator are very small.

[0068] Next, magnesium oxide was applied to an magnetic steel sheet toexamine the properties of a forsterite film. Steel to be examined is asteel sheet obtained by subjecting to hot rolling, washing with an acid,and cold rolling by a known method a silicon steel slab forgrain-oriented magnetic steel sheet, which slab comprises C: 0.058%; Si:2.8%; Mn: 0.06%; Al: 0.026%; S: 0.024%; N: 0.0050%, in terms of % bymass; and the balance of unavoidable impurities and Fe, so that thefinal sheet thickness becomes 0.23 mm, and subjecting the resultantsheet to decarburization annealing in a wet atmosphere comprised of 25%of nitrogen gas and 75% of hydrogen gas.

[0069] The magnesium oxide particle aggregates of the present inventionand the magnesium oxide particles in Comparative Examples, each in aslurry form, were individually applied to the above steel sheet so thatthe dried coating weight became 12×10⁻³ kg·m⁻², and dried and then,subjected to final finishing annealing at 1,200° C. for 20 hours. Thestates of the individual forsterite films formed on the steel sheets areshown in Table 4. TABLE 4 State of forsterite film formed State of glassfilm formed Evaluation Example 1 Uniform and ⊚ thick Example 2 Uniformand ⊚ thick Example 3 Uniform and ⊚ thick Comparative Example 1 Uniformand ⊚ thick Comparative Example 2 Uniform and ◯ slightly thinComparative Example 3 Uniform and ◯ slightly thin Comparative Example 4Nonuniform and Δ slightly thin Comparative Example 5 Nonuniform and Δslightly thin Comparative Example 6 Nonuniform and X very thin

[0070] As can be seen from Table 4, the forsterite films formed from theparticle aggregates in Examples 1 to 3 and Comparative Example 1 arethose having a uniform and satisfactory thickness. Especially theforsterite films formed from the particle aggregates in Examples 1 to 3have been confirmed to have a large thickness and excellent adhesionproperties, as compared with the forsterite films formed from themagnesium oxide particles which are currently used as annealingseparators for high grade grain-oriented magnetic steel sheets.

INDUSTRIAL APPLICABILITY

[0071] As mentioned above, in the present invention, there can beprovided magnesium oxide having a particle aggregation structure whichcan advantageously form forsterite. In addition, the magnesium oxideparticle aggregate of the present invention exhibits excellentforsterite formation, as compared with the magnesium oxide currentlyused as an annealing separator for grain-oriented magnetic steel sheets.Therefore, the grain-oriented magnetic steel sheet obtainable by atreatment using the magnesium oxide of the present invention hassatisfactory magnetic properties as a grain-oriented magnetic steelsheet. Further, the technical concept of the present invention can beapplied not only to the annealing separator but also to other solidphase reactions, for example, ceramic synthesis.

1. A magnesium oxide particle aggregate, characterized in that a firstinflection point diameter is more than 0.30×10⁻⁶ to 0.60×10⁻⁶ m, aparticle void volume is 0.50×10⁻³ to 0.90×10⁻³ m³·kg⁻¹, and a microporevolume is 0.04×10⁻³ to 0.11×10⁻³ m³·kg⁻¹ in the cumulative intrusionvolume curve of said particle aggregate.
 2. A magnesium oxide particleaggregate, characterized in that an interparticle void volume is0.80×10⁻³ to less than 1.40×10⁻³ m³·kg¹, a particle void volume is0.50×10⁻³ to 0.90×10⁻³ m³·kg⁻¹, and a micropore volume is 0.04×10⁻³ to0.11×10⁻³ m³·kg⁻¹ in the cumulative intrusion volume curve of saidparticle aggregate.
 3. An annealing separator for grain-orientedmagnetic steel sheet, using the magnesium oxide particle aggregateaccording to claim 1 or
 2. 4. A grain-oriented magnetic steel sheetobtainable by a treatment using the annealing separator forgrain-oriented magnetic steel sheet according to claim 3.